FR3091881A1 - Static penetrometer with a remote compressible system and use of such a penetrometer - Google Patents

Abstract

The invention relates to a penetrometer (100) comprising: at least one central rod (1) terminated at a first end by a measuring tip (11); At least one hollow tube (2) surrounding the central rod (1), the latter being able to slide inside the hollow tube (2); A measuring cell (3) in contact with the hollow tube (2), intended to transmit a force applied by support means, so as to cause the hollow tube (2) and the central rod to sink into the ground (1); The penetrometer (100) is remarkable in that it comprises a compressible system (6) providing an elastic connection between the measuring cell (3) and a second end (12) of the central rod (1), said compressible system ( 6) comprising at least one column (63) offset from the axis of the central rod (1). Figure to be published with the abstract: Figure 1

Description

static penetrometer with remote compressible system and use of such a penetrometer

The present invention relates to the field of geotechnics and geology. It relates in particular to a device for measuring the resistance to penetration of a soil, commonly called a penetrometer, and associated measuring methods.

TECHNOLOGICAL BACKGROUND OF THE INVENTION

The compactness of soils is usually measured, either in static mode or in dynamic mode, by a measuring device called a penetrometer. A penetrometer conventionally comprises rods connected end to end to form a string of rods at the end of which is fixed a measuring tip, intended to sink into the ground to depths that can reach several tens of meters.

In static mode, the drill string is pushed by cylinders, causing the progressive sinking of the measuring tip; the latter measures the tip resistance and optionally the lateral friction on a cylindrical sleeve located above the tip. These measurements are recorded continuously or discontinuously according to a regular step. The static measurement of the resistance to soil penetration is unquestionably the most precise because it is carried out directly on the measuring tip, at the bottom of the borehole.

This method of static measurement nevertheless comes up against some difficulties. When the resistance of the soil is high (very compact soils), it becomes very complex to apply sufficient thrust to generate the tip sinking. Moreover, when the measuring tip and the drill string are driven several meters deep, the friction exerted by the ground on the drill string must also be overcome by a strong push to continue driving. As a result, the weight, power and quality of the anchoring of the machines that operate the jacks are sometimes not sufficient to ensure a measurement in static mode in all circumstances.

There is also another measurement mode called dynamic mode based on the sinking of the tip by beating the drill string. Driving is done by dropping a sledgehammer or hammer onto an anvil driving the drill string in jerks. Knowing the energy released by the fall of the mass and knowing the jerk penetration, the resistance to soil penetration is calculated, taking into account various coefficients of energy loss. This dynamic mode has a greater penetration capacity than the static mode, especially in soils of high compactness. A measurement in dynamic mode is nevertheless much less precise than a measurement in static mode and gives only partial information of the resistance of the ground according to the depth, because of the sinking by jerks and in general of the non-dissociation of the peak resistance with respect to parasitic lateral friction.

The document FR2584186 describes a device for measuring soil characteristics by dynamic penetration comprising a string of solid rods ending in a point and sliding inside a string of hollow rods, and a threshing head rigidly fixed to the hollow rods and elastically secured to the solid rods by the interposition of a spring. The fall of a hammer on the threshing head causes the drill strings to sink. During the lifting and the new fall of the hammer (duration 0.3 to 5 seconds), the spring relaxes and the tip continues to sink until the reaction of the ground is equal to the compression force of the spring . This device has the disadvantage of making, between two dynamic depressions by the fall of a hammer on the threshing head, only occasional and discontinuous measurements in static mode, at random depths depending on the resistance of the soil.

Other so-called dynamic static penetrometers are capable of working in mixed dynamic and static mode, as soon as the all static mode is no longer possible, typically in the presence of soil layers with high compactness, the penetration resistance becoming too important.

The document EP2535460 is known in particular, which describes a device comprising a measuring rod terminated by a measuring tip, a protective tube arranged around the measuring rod, lowering and lifting means for effecting the sinking into the ground in static mode, striking means for effecting the driving into the ground in dynamic mode and measuring means for determining the resistance to penetration of the ground. The particular construction of this device makes it possible to isolate the dynamic depression of the external protection tube, from the static measurement tapping of the resistance at the tip and from the lateral friction through the internal measuring rod. Thus the measuring rod is not subjected to the mechanical forces exerted by the striking means and the measurement can be carried out in static mode.

The structure of this device nevertheless remains complex and cumbersome to implement, both from the point of view of the device itself and of the related equipment necessary to move and use said device. In addition, the beating phase does not allow the measurement of the resistance at the tip (the latter not being dissociated from the lateral friction of the sleeve), and this identically to the device described in the document FR2584186, so that certain important features of the terrain may escape the knowledge of the geologist.

OBJECT OF THE INVENTION

An object of the present invention is to propose an alternative solution to the solutions of the state of the art, in particular a static penetrometer simple to implement, robust and allowing precise and continuous measurements of the compactness of a soil.

BRIEF DESCRIPTION OF THE INVENTION

• Au moins une tige centrale terminée à une première extrémité par une pointe de mesure ;
• Au moins un tube creux entourant la tige centrale, cette dernière étant apte à coulisser à l’intérieur du tube creux ;

• Une cellule de mesure en contact avec le tube creux, destinée à transmettre une force appliquée par des moyens d’appui, de manière à provoquer un enfoncement en mode statique dans le sol du tube creux et de la tige centrale ;

The invention relates to a penetrometer comprising:

• At least one central rod terminated at a first end by a measuring tip;
• At least one hollow tube surrounding the central rod, the latter being able to slide inside the hollow tube;

• A measurement cell in contact with the hollow tube, intended to transmit a force applied by bearing means, so as to cause a depression in static mode in the ground of the hollow tube and of the central rod;

• Une première chambre à huile formée dans la cellule de mesure, et un premier piston solidaire d’une seconde extrémité de la tige centrale et apte à coulisser dans la première chambre ;
• Au moins une colonne déportée par rapport à l’axe de la tige centrale, qui comprend une deuxième chambre à huile en communication fluidique avec la première chambre, un deuxième piston apte à coulisser dans la deuxième chambre et un dispositif compressible étalonné en contact avec le deuxième piston ;
• Un capteur de pression relié à la deuxième chambre et/ou un capteur de déplacement apte à mesurer la longueur du dispositif compressible étalonné.

The penetrometer is remarkable in that it comprises a compressible system providing an elastic connection between the measuring cell and a second end of the central rod, said compressible system comprising:

• A first oil chamber formed in the measuring cell, and a first piston integral with a second end of the central rod and able to slide in the first chamber;
• At least one column offset with respect to the axis of the central rod, which comprises a second oil chamber in fluid communication with the first chamber, a second piston able to slide in the second chamber and a calibrated compressible device in contact with the second plunger;
• A pressure sensor connected to the second chamber and/or a displacement sensor capable of measuring the length of the calibrated compressible device.

• le capteur de pression est apte à mesurer en continu la pression dans la deuxième chambre, au cours de l’enfoncement ;
• le capteur de déplacement est apte à mesurer en continu la longueur du dispositif compressible étalonné, au cours de l’enfoncement ;
• la communication fluidique entre la première chambre et la deuxième chambre est faite au moyen d’un tuyau flexible ;
• la communication fluidique entre la première chambre et la deuxième chambre est faite au moyen d’un conduit rigide ;
• la cellule de mesure comporte une partie interne, une partie externe et un espace annulaire étanche entre la partie interne et la partie externe ;
• la première chambre est formée dans la partie interne et est apte à communiquer avec l’espace annulaire via un premier orifice ;
• le conduit rigide est fixé à la partie externe qui comprend un deuxième orifice pour communiquer avec l’espace annulaire ;
• la colonne déportée comprend une vis de réglage configurée pour faire varier le volume de la deuxième chambre en se déplaçant dans la colonne déportée ;
• la vis de réglage est actionnée par un moteur asservi et piloté par un contrôleur électronique ;
• le pénétromètre comprend des moyens supplémentaires d’enfoncement aptes à appliquer au tube creux, une vibration à une fréquence déterminée ;
• les moyens supplémentaires d’enfoncement sont couplés à la cellule de mesure ;
• les moyens supplémentaires d’enfoncement sont composés par au moins un dispositif hydraulique ou électrique de vibro-fonçage ;
• le pénétromètre comprend un contrôleur électronique configuré pour enregistrer les mesures issues du ou des capteurs et pour activer ou désactiver les moyens supplémentaires d’enfoncement ;
• le dispositif compressible est formé par un ressort étalonné ou par une pluralité de rondelles Belleville ;
• le pénétromètre comprend les moyens d’appui aptes à appliquer une force à la cellule de mesure, pour effectuer un enfoncement dans le sol en mode statique du tube creux et de la tige centrale ;
• le pénétromètre comprend un châssis muni d’au moins un élément de liaison mécanique destiné à être connecté à un massif de réaction ;
• la course de la pointe de mesure par rapport à une extrémité du tube creux est définie par la variation du volume de la deuxième chambre et adaptée au massif de réaction.

According to other advantageous and non-limiting characteristics of the invention, taken alone or according to any technically feasible combination:

• the pressure sensor is capable of continuously measuring the pressure in the second chamber, during depression;
• the displacement sensor is capable of continuously measuring the length of the calibrated compressible device, during depression;
• the fluid communication between the first chamber and the second chamber is made by means of a flexible pipe;
• the fluid communication between the first chamber and the second chamber is made by means of a rigid pipe;
• the measurement cell comprises an internal part, an external part and a sealed annular space between the internal part and the external part;
• the first chamber is formed in the internal part and is capable of communicating with the annular space via a first orifice;
• the rigid conduit is fixed to the external part which comprises a second orifice to communicate with the annular space;
• the offset column comprises an adjustment screw configured to vary the volume of the second chamber by moving in the offset column;
• the adjustment screw is actuated by a motor controlled and controlled by an electronic controller;
• the penetrometer comprises additional driving means capable of applying to the hollow tube, a vibration at a determined frequency;
• the additional driving means are coupled to the measuring cell;
• the additional driving means are made up of at least one hydraulic or electric vibro-driving device;
• the penetrometer comprises an electronic controller configured to record the measurements from the sensor(s) and to activate or deactivate the additional driving means;
• the compressible device is formed by a calibrated spring or by a plurality of Belleville washers;
• the penetrometer comprises support means capable of applying a force to the measuring cell, to effect a driving into the ground in static mode of the hollow tube and of the central rod;
• the penetrometer comprises a frame provided with at least one mechanical connection element intended to be connected to a reaction mass;
• the stroke of the measuring tip relative to one end of the hollow tube is defined by the variation in the volume of the second chamber and adapted to the reaction mass.

• une utilisation pour la mesure de la résistance à la pénétration d’un sol,
• une utilisation pour l’évaluation du caractère liquéfiable d’un sol,
• une utilisation pour la mesure des propriétés élasto-plastiques d’un sol.

The invention also relates to various uses of the penetrometer as above:

• use for measuring the penetration resistance of a soil,
• use for evaluating the liquefiable nature of a soil,
• use for measuring the elastoplastic properties of a soil.

Other characteristics and advantages of the invention will emerge from the detailed description of the invention which will follow with reference to the appended figures in which:

Figure 1 shows a block diagram of a penetrometer according to a first embodiment of the invention;

Figure 2 shows a block diagram of a penetrometer according to a second embodiment of the invention.

In the descriptive part, the same references in the figures may be used for elements of the same type. The figures are schematic representations which, for the purpose of readability, are not necessarily to scale.

The present invention relates to a penetrometer 100 which can be used in particular for the measurement in static mode of the resistance to penetration of a soil.

The penetrometer 100 comprises at least one central rod 1 terminated at a first end by a measuring tip 11. By way of example, a well-known measuring tip of the “Gouda” type could be mounted at the level of the first end of said center rod 1.

The penetrometer 100 comprises at least one hollow tube 2 surrounding the central rod 1. The respective diameters of the hollow tube 2 and of the central rod 1 are adapted so that the latter can slide freely inside the hollow tube 2.

The couple formed by the central rod 1 and the hollow tube 2 is intended to sink into the ground, the point 11 at the end having the function of measuring the resistance to penetration of the ground. To test the resistance of the ground at various depths, additional rods 1 and tubes 2 can be connected end to end, to form a train of rod/tube pairs, which can be driven into the ground to depths of several tens of meters.

The penetrometer 100 also comprises a measuring cell 3 in contact with the hollow tube 2. The measuring cell 3 is intended to transmit a force F applied by support means, so as to cause a depression in static mode in the ground of the couple formed by the hollow tube 2 and the central rod 1. The support means (not shown) capable of applying said force F will be described later.

The measuring cell 3 may in some cases be integral with the hollow tube 2, in particular if the friction of the hollow tube 2 in the ground is used as a reaction element. In cases where the measuring cell 3 is only in contact with the hollow tube 2, it is the thrust by the support means which maintains the mechanical continuity between cell 3 and tube 2. An adapter at the head of the hollow tube 2 can allow indifferent passage from one mode to another.

The penetrometer 100 further comprises a compressible system 6 ensuring an elastic connection between the measuring cell 3 and a second end 12 of the central rod 1 (or of the train of central rods). The second end 12 is located opposite the first end which supports the measuring tip 11.

As illustrated in Figures 1 and 2, the compressible system 6 comprises a first oil chamber 61 formed in the measuring cell 3, and a first piston 62 integral with the second end 12 of the central rod 1; the first piston 62 is able to slide in the first chamber 61. In particular, when the measuring tip 11 is fully extended relative to the hollow tube 2, the first piston 62 will be in the low position (according to the arrangement of FIGS. 1 and 2 ) in the first chamber 61; when the measuring tip 11 is erased, that is to say resting against the hollow tube 2, the first piston 62 will be in a higher position in the first chamber 61. The first piston 62 is therefore able to slide in the first chamber 61 depending on the reaction force that the ground applies to the measuring tip 11.

The compressible system 6 also comprises a column 63 offset with respect to the axis of the central rod 1. The offset column 63 comprises a second oil chamber 64 in fluid communication with the first chamber 61 and a second piston 65 capable of sliding in the second chamber 64. The compressible system 6 also includes a calibrated compressible device 66 in contact with the second piston 65.

According to a first embodiment illustrated in Figure 1, the fluid communication between the first chamber 61 and the second chamber 64 is made by means of a flexible pipe 614a, which allows the offset at a chosen distance of the column 63 with respect to to the measuring cell 3 / tube train 2 & rod 1 assembly and to the driving axis.

According to a second embodiment illustrated in Figure 2, the fluid communication between the first chamber 61 and the second chamber 64 is made by means of a rigid conduit 614b and relatively short, which also allows an offset of the column 63 with respect to to the measuring cell 3 / tube train 2 and rod 1 assembly, but more limited than in the first embodiment. Since the compressible device 66 can be a bulky element of column 63, it is advantageously offset with respect to column 63, in a second column 63': second chamber 64 is then extended by a second conduit 64b, advantageously flexible and of appropriate, up to the piston 65 arranged in the second column 63', in contact with the compressible device 66.

We will see later that the first or the second mode of implementation can be more advantageous depending on the type of test to be carried out.

The displacement of the first piston 62 in the first chamber 61 according to the reaction force that the ground applies to the measuring tip 11, will be transmitted to the second piston 65 in the second chamber 64, due to the low compressibility of the oil present in and between the two chambers. The second piston 65 will then apply a load on the calibrated compressible device 66, capable of compressing or allowing said device 66 to extend.

In the compressible system 6 of the penetrometer 100 according to the invention, the length of the calibrated compressible device 66 can advantageously vary from 40 to 100 mm between its elongated state and its completely retracted state. Note that the stiffness of the compressible device 66 can be adapted to the hardness of the ground envisaged after preliminary investigation. In particular, it is possible to choose the stiffness of the compressible device 66 so that it passes from its elongated state (for a zero load) to its completely retracted state for an applied load of between 100 and 500 bars.

According to a first variant, the compressible device 66 consists of a calibrated spring. The length of the spring varies linearly with the load applied to it.

According to a second variant, the calibrated compressible device 66 consists of a plurality of Belleville washers, mounted in opposition. The deformation of such an assembly is almost linear, and its stiffness can be defined as: , where n is the number of washers, P is the flattening load of a washer, beyond which the washer practically no longer deforms, and h ₀ is the maximum deflection. This variant is advantageous in that the stiffness of the assembly of washers can be easily adjusted by adding or removing washers, which can be assembled in a wide variety of arrangements: in opposition or in series or even in one combination of these provisions, and in varying proportions. The use of a plurality of Belleville washers is particularly advantageous in that it allows a vertical space requirement, in the offset column 63, which is smaller than when a spring is used. Identically to the first variant, the load applied to the plurality of Belleville washers is also related to the length of the compressible device, corresponding to the flattening (retraction) and/or the expansion (elongation) of the assembly of washers .

The calibrated compressible device is of course not limited to the two variants mentioned above and could alternatively be formed by a gas spring or any elastic element (elastomer, etc.), allowing retraction and elongation of the order of magnitude cited above, for loads applied as stated above.

The compressible system 6 according to the invention further comprises a pressure sensor 67 connected to the second chamber 64 suitable for measuring the pressure in said second chamber 64. Taking into account the surface ratios between the tip 11, and the first 62 and second 65 pistons, it is thus possible to extract from the measurement made by the pressure sensor 67, the pressure exerted at the level of the tip 11, representative of the resistance of the ground.

Alternatively, the compressible system 6 may comprise a displacement sensor 68, instead of or in addition to the aforementioned pressure sensor 67. This displacement sensor 68 is capable of measuring the length of the calibrated compressible device 66 during its elongation or its retraction, during the sinking of the tube 2 / rod 1 pair into the ground. This length l is representative of the resistance of the ground, to which it can be linked by a relation of proportionality: , where k is the stiffness of the compressible device 66 and R the resistance of the ground.

The penetrometer 100 according to the invention, equipped with the compressible system 6, therefore makes it possible to measure the resistance of the ground continuously, as the tube 2 / rod 1 couple is driven into the ground in static mode. Advantageously, the sensor(s) 67,68 comprise(s) a recording system to collect the measurements continuously during the depression. By way of example, the pressure sensor 67 can take a measurement every tenths of a second or even every second of the pressure in the second chamber 64.

The compressible system configuration 6 with offset column 63 makes it possible to lighten and simplify the assembly made up of the measuring cell 3 and the tube train 2 / rod 1, which must be maintained along the driving axis (parallel to the axis of the drill string). Indeed, the alternative to this configuration, which would consist in positioning the calibrated compressible device 66 directly between the measuring cell 3 and the central rod 1, requires said device 66 to be maintained in the driving axis and increases the size of the penetrometer.

Advantageously, the offset column 63 comprises an adjustment screw 69 configured to vary the volume of the second chamber 64 by moving in the offset column 63. Preferably, the adjustment screw 69 is actuated by a motor 70 controlled and controlled by an electronic controller. The movement and the speed of movement of the adjustment screw 69 can thus be controlled. It should be noted that by adjusting screw is meant any means actuated manually or automatically, capable of performing a translational movement in the column 63 so as to modify the volume of the second chamber 64. For example, a cylinder piston could also correspond to said adjusting screw 69.

By varying the volume of the second chamber 64, the screw 69 will more or less compress the calibrated compressible device 66 and more or less apply pressure on the central rod 1 (via the first piston 62) due to the communication between the rooms 64.61. The adjustment screw 69 thus makes it possible to adapt the stroke of the tip 11, between an extended position and its retracted position, according to the reaction mass used. The reaction block is described below in conjunction with the bearing means 4 for driving in static mode the tube 2 / rod 1 couple.

Preferably, the static penetrometer 100 according to the invention also comprises support means 4, suitable for applying a force F to the measuring cell 3, and making it possible to carry out a sinking into the ground in static mode of the torque formed by the hollow tube 2 and the central rod 1. The force F is applied to an outer part 3b of the cell 3.

The support means may in particular comprise at least one hydraulic cylinder capable of developing a power of between 5 and 15 tons. A part 41 of the cylinder is fixed to the outer part 3b of the cell 3 and applies thereto the force necessary for the continuous depression of the pair tube 2 / rod 1. Another part 42 of the cylinder must be fixed directly or indirectly to a massive reaction of 200.

The support means 4 may include a self-propelled hydraulic unit, to actuate the cylinder 41,42.

Advantageously, the support means 4 are held by a frame 5. The frame 5 is provided with at least one mechanical connection element intended to be connected to a reaction mass 200. This mechanical connection element may for example consist of a hydraulic or mechanical clamp, or even a vice of the same type. The fact that the chassis 5 is equipped with such a mechanical connection element makes it connectable to any kind of reaction mass 200: crane, mechanical shovel, heavy goods vehicle, etc.

Preferably, the penetrometer 100 comprises additional driving means 8 able to apply to the hollow tube 2, a vibration at a determined frequency. We will see later that this vibration, applied temporarily during the sinking when a high resistance ground is encountered, helps to continue the sinking in static mode, without requiring an oversized reaction block 200 or the use of a dynamic assistance that prevents accurate measurement of ground resistance.

Advantageously, these additional means 8 are coupled to an internal part 3a of the cell 3 which is integral with the hollow tube 2. The vibration applied to the internal part 3a is thus transmitted to the hollow tube 2.

The first chamber 61 is formed in this internal part 3a. Provision is made for the internal part 3a to be able to move along the axis of insertion into the external part 3b of the measuring cell 3. When the vibration is applied to the internal part 3a, the latter is thus free to oscillate in limiting the transmission of the vibration to the outer part 3b secured to the support means 4.

In the first embodiment illustrated in FIG. 1, the flexible pipe 614a is directly connected to the first chamber 61 via a first orifice provided in the internal part 3a. A second orifice arranged in the external part 3b of the cell 3 allows the flexible pipe 614a to connect the second chamber 64 and will leave sufficient latitude to accommodate the oscillation of the internal part 3a when a vibration is applied to it.

In the second embodiment illustrated in FIG. 2, seals 3c are arranged between the internal part 3a and the external part 3b of the cell 3 and define a sealed annular space 3d between the two parts 3a, 3b. A first orifice in the internal part 3a establishes fluid communication between the first chamber 61 and the annular space 3d; a second orifice in the outer part 3b establishes fluid communication between the annular space 3d and the duct 614b, which connected to said second orifice, provides communication with the second chamber 64. This fluid connection configuration prevents the rigid duct 614b from undergoing the vibrations applied to the internal part 3a by the additional means 8, likely to degrade it. Such a configuration could also be implemented in the embodiment illustrated in FIG. 1, in which the compressible system 6 comprises a flexible conduit 614a, but is of less interest, the flexible conduit 614a being capable of withstanding vibrations.

The penetrometer according to the invention can therefore resort to vibro-dynamic assistance, while maintaining a depression in static mode and a measurement via the compressible system 6 which is continuous and precise, representative of the hardness of the ground.

By way of example, the additional driving means 8 are composed of at least one hydraulic, pneumatic or electric device (or other source of energy) for vibro-sinking, providing mechanical energy by pulse, at a determined frequency. Preferably, the determined frequency of the vibration is between 35 and 100 Hz. And the unit energy, that is to say at each impulse or blow, developed by the additional driving means is between 25 and 400 Joules .

It should be noted that the configuration of compressible system 6 with offset column 63 makes said compressible system 6 more reliable, the mechanical parts of which (in particular the calibrated compressible device 66) are not directly subjected to the vibration likely to be applied for the sinking of the train of tubes 2/rod 1, since they are arranged in the offset column 63 (or the second offset column 63').

The penetrometer 100 may further comprise an electronic controller capable of analyzing the measurements from the sensor(s) 67, 68, of actuating the adjustment screw 69 (for example, via the motor 70), and of activating or deactivating the additional means of depression 8, according to criteria programmed or defined by an operator during the test.

Measurement of soil resistance in static mode

The penetrometer 100 according to the present invention can be used for measuring the resistance to penetration of a soil, in particular according to the measurement method in static mode described below.

To carry out a test, the frame 5 of the penetrometer 100 is connected to a reaction block 200. The maximum bearing force that can be applied to the measuring cell 3 for the penetration of the tube 2 / rod 1 couple is defined by the mass of said reaction mass 200. If, during the test, the tip 11 encounters a ground whose resistance exceeds the maximum bearing force, the penetrometer 100 is able to resort to vibro-dynamic assistance thanks to the means additional depression 8. To avoid damaging the rod 1, it is important that the tip 11 is in the retracted position (that is to say resting against the hollow tube 2) when the vibro-dynamic assistance is Implementation.

The adjustment screw 69 of the compressible system 6 makes it possible to adapt the stroke of the point 11 to the reaction mass 200, so that said point 11 is in the retracted position when the resistance of the ground reaches the maximum bearing force.

Following this adjustment, the measurement method firstly comprises driving into the ground in static mode (continuous and regular driving) of the couple formed by the hollow tube 2 and the central rod 1, at a driving speed given. The driving speed in static mode is between 1 and 5 cm/s; advantageously, to comply with the measurement standards in force, the driving speed is of the order of 2cm/s.

During this depression, the method provides for the measurement, by the pressure sensor 67, of the pressure in the second chamber 64 of the compressible system 6. This pressure measurement makes it possible to go back to the peak resistance of the ground (Qc), for successive depths of penetration into the ground.

When the measuring tip 11 encounters a very resistant ground, for which the maximum support force is reached, the tip 11 finds itself in its retracted position (that is to say resting against the hollow tube 2). The method then provides for the application, by the additional driving means 8, of vibro-driving at a determined frequency. As mentioned above, the determined frequency is preferably between 35 and 100 Hz, and the unit energy (per pulse) is between 25 and 400 Joules. The applied vibration, transmitted to the hollow tube 2, either directly or via the internal part 3a of the measuring cell 3, makes it possible to help the crossing of particularly hard ground layers.

The application of a vibration thanks to the additional driving means makes it possible to cross this layer, until reaching a layer of lesser hardness. At this time, the compressible device 66 will lengthen, the measuring tip 11 at the end of the rod 1 encountering a lower resistance. The additional depression means 8 are then deactivated and the depression in static mode resumes.

The method according to the invention thus makes it possible to measure the resistance of the ground continuously and without interruption because the crossing of the hard layers in depth is done gradually, by vibro-sinking. As soon as the hardness of the ground decreases, the measurement by the sensor(s) 67,68 of the compressible system 6 makes it possible to obtain a resistance value of the ground. No information is thus lost on the characteristics of the ground, which are measured continuously via the pressure variations in the second chamber 64 of the compressible system 6 and/or via the variations in length of the calibrated compressible device 66.

Advantageously, an electronic controller is configured to record the measurements from the sensor(s) (pressure 67 and/or displacement 68) and to activate or deactivate the additional driving means 8. It can also be configured to control the means of support 4 and enslave the force F applied to the measuring cell 3, at the speed at which the tube 2 / rod 1 pair is driven into the ground.

It should be noted that the first and the second mode of implementation of the penetrometer 100 (FIGS. 1 and 2) are suitable for carrying out the present method for measuring the resistance of a soil in static mode.

Evaluation of the liquefiability of a soil

The penetrometer 100 according to the present invention can also be used for evaluating the liquefiable nature of a soil, in particular by applying the method described below.

The method comprises a first step a) consisting in driving the tube 2 / rod 1 pair into the ground to bring the measuring tip 11 to a given depth for the investigation of the liquefiable nature. The support means 4, via the cell 3, make it possible to carry out this depression at the given depth.

After the measuring tip 11 has reached the given depth, it is important that it is not in its fully extended position: the stroke of the tip 11 can then be adapted via the adjustment screw 69 of the system compressible 6 (by varying the volume of the second chamber 64).

From there, the calibrated compressible device 66 is clamped, so as to remain at constant length for the following stages of the method.

A second step b) of the method consists in inducing, from the given depth, a first controlled sinking of the measuring tip 11 into the ground. The calibrated compressible device 66 being clamped, the actuation of the adjustment screw 69 by the servo-controlled and controlled motor 70 makes it possible to reduce the volume of the second chamber 64 and thus to increase the pressure against the first piston 62 to induce a first displacement of the rod 1 (corresponding to the first controlled depression of the tip 11). For example, the first depression is 10mm and is carried out at a constant speed of 2cm/s.

The pressure applied (pressure measured via the pressure sensor 67) to perform said first depression is representative of the resistance of the ground and gives a peak resistance . Preferably, after the first depression, the pressure in the second chamber 64, when stopped (representative of the peak resistance when stopped ) is also measured.

Values and extracted from the measurements in step b) constitute the starting point of the test for evaluating the liquefiable nature of the soil layer investigated.

A third step c) of the method consists in applying a low-frequency vibration to the second end 12 of the central rod 1 and simultaneously causing it to perform a second movement inducing a second controlled sinking of the tip 11 into the ground. Such a low frequency vibration is applied via the adjustment screw 69, which by gradually decreasing the volume of the second chamber 64 by back and forth movements at the chosen frequency, will apply the vibration and generate the second depression.

The chosen low frequency is between 1 and 5 Hertz, frequencies characteristic of earthquakes. The objective here is to locally apply stresses likely to modify the properties of the soil layer, as an earthquake could do. The second insertion of the tip 11 into the soil layer takes place while it is under vibratory stress. For example, the second recess may be 30mm.

The pressure applied to perform the second depression is representative of the peak resistance during this second controlled depression.

Advantageously, the speed of the second depression is adjusted so as to maintain the pressure applied substantially constant, thanks to a servo-control of the motor actuating the adjustment screw 69. The second depression is therefore advantageously carried out at a constant load (force applied). In particular, an applied force is aimed at substantially equal to the value of applied force representative of the peak resistance measured in step b). The speed of the second depression is therefore automatically increased or decreased depending on the pressure measured during step c), with the aim of keeping it substantially constant. The speed at which the second depression is carried out, at constant force, gives an important indication of the sudden nature of the loss of resistance of the soil layer under vibratory stress.

Several scenarios may arise, depending on the properties of the soil layer analyzed. The resistance properties of the soil layer can change very slowly under vibratory stress: the second depression then takes place at a low speed. In such a case, it appears that the soil layer does not change suddenly with a vibratory stress. According to another scenario, the layer of soil can very quickly lose its resistance under the vibratory stress: the second depression then takes place at a high speed. The risks in the event of an earthquake in the two scenarios mentioned are clearly different: the precautions and specifications on the foundations of constructed structures can be adapted to each case.

When the second controlled depression is completed the vibration is stopped.

The fourth step d) of the method provides for actuating the adjustment screw 67 so as to cause a third controlled sinking of the measuring tip 11 into the ground, preferably at constant speed.

For example, the third depression could be 10mm, carried out at a speed of 2cm/s.

During step d), in the absence of vibratory stress, the soil layer can see its resistance evolve in different ways, depending on the characteristics of said layer.

According to a first behavior, a layer having quickly lost its resistance under vibratory stress can, in the absence of the latter, regain its initial resistance. : it is a liquefiable soil behavior.

According to a second behavior, a layer having quickly lost its resistance under vibratory stress may, in the absence of the latter, return during the third controlled depression to a resistance lower than its initial strength : this may reflect a phenomenon of the large deformation type, the resistance properties of the soil layer having been irreversibly modified by the vibration.

Finally, according to a third behavior, a layer having quickly lost its resistance under vibratory stress may, in the absence of the latter, return during the third controlled depression to a resistance greater than its initial strength : this can reflect a phenomenon of densification, the resistance of the soil layer having been reinforced by the vibration.

Steps a) to d) can be repeated for other given investigation depths, so as to analyze successive soil layers included in a suspect zone of greater or lesser thickness.

The aforementioned method and the penetrometer 100 according to the invention thus make it possible to evaluate the liquefiable nature of the layer of soil analyzed, from the speed of realization of the second depression. The more or less sudden decrease in the resistance of the soil under vibratory stress is a key criterion of the liquefiable character. It is also possible to derive important information on the bearing properties of the soil layer analyzed, following a vibratory stress, making it possible to anticipate potential irreversible changes in the resistance of the soil.

Note that the penetrometer 100 according to the second mode of implementation (FIG. 2) will preferably be used to carry out this method, the rigid and short fluidic communication between the second chamber 64 and the first chamber 61 being more suitable for an effective application of the cyclic stress (low frequency vibration) to piston 62 (in connection with rod 1), by adjusting screw 69.

Measurement of properties elasto -plastic of a floor

The penetrometer 100 according to the present invention can also be used for measuring the elasto-plastic properties of a soil, in particular by applying the method described below.

The method firstly comprises a first step a) consisting in driving the pair formed by the hollow tube 2 and the central rod 1 into the ground to bring the measuring tip 11 to a desired depth for the measurement of the elasto-plastic properties. of the ground. Arrived at said depth, the measuring tip 11 is erased, that is to say placed in abutment against the hollow tube 2, by means of the adjustment screw 69 (that is to say by varying the volume of the second bedroom 64).

From there, the calibrated compressible device 66 is clamped, so as to remain at constant length for the following stages of the method.

The method comprises a second step b) during which a first constant force F ₁ is applied to a second end 12 of the rod 1, for a determined duration t. Remember that a force can be applied to the first piston 62 in connection with the rod 1, via the adjustment screw 69 which can vary the pressure in the second chamber 64 and in the first chamber 61 (the two chambers 61,64 being in fluid communication).

The constant force F ₁ applied is transmitted by the rod 1 to the measuring tip 11, which will more or less sink into the ground, depending on the mechanical characteristics of the latter.

The first force F ₁ applied may be between 100N and 500N; considering a section of 10 cm2 of the measuring tip 11, this corresponds to an applied stress of between 1 and 5 bars.

In a third step c) of the method, a first final displacement is measured , associated with the insertion of the measuring tip 11 into the ground, after the determined duration t. A displacement corresponding to the displacement noted at the end of the determined duration t is qualified here as “final”.

The method then provides for the repetition of steps b) and c), with increasing applied forces, to form a curve representing the final displacement as a function of the applied force. Thus, a second constant force F ₂ , greater than F ₁ , is applied to the second end 12 of the rod 1, for the determined duration t. A measurement is made of the second final displacement , associated with the insertion of the measuring tip 11 into the ground, after the determined duration t. Note that this second final move corresponds to the accumulation of the first final move and the additional displacement caused by the application of the second force F ₂ . A third constant force F ₃ , greater than F ₂ , is then applied, still for the determined duration t, then a measurement of the third final displacement is carried out at the end of said duration t, and so on.

Advantageously, the increment between two successive applied forces is between 100 and 500N.

The determined duration t may vary between 15 and 600 seconds; it is preferably defined at t = 60s because this duration is consistent with the methodological approach of known pressuremeter tests.

The sequence of application of a force and measurement of the final displacement is repeated n times (application of an nth constant force F _n , greater than F _n-1 , and measurement of an nth final displacement ) until reaching the breaking stress of the soil Qc. The breaking stress of the soil will correspond to a force F _n generating a final displacement maximum due to significant penetration of the measuring tip 11. Recall that the force F _n is linked to the stress by the relationship: Qc = F _n / S, S being the surface of the plane section of the measuring tip 11 ( for example 10cm ² ).

In practice, the maximum displacement representative of the rupture of the ground is defined at around 5 cm (output amplitude of the point 11 with respect to its retracted position against the hollow tube 2).

At the end of the previous steps, we can draw a curve representing the final displacement as a function of the applied force. The curve is formed of a first domain corresponding to elastic deformations of the ground. It is formed, after a point of inflection, of a second domain corresponding to plastic deformations of the ground, until reaching the breaking point (Qc).

The method then includes the extraction of the deformation modulus M of the soil from the slope of the final displacement / force curve, in the first domain of elastic deformation located before the point of inflection.

The slope p of the curve in the first elastic domain E is expressed:

(note that and F ₃ are given here by way of example, the principle being to use a final displacement / force couple, in the elastic domain, making it possible to calculate precisely the slope of said domain)

The deformation modulus M, in MPa/m, is calculated from the expression: with p the slope of the first elastic domain and S the surface of the plane section of the measuring tip 11. The modulus thus extracted allows the modeling of the settlement of the ground under load.

The method also makes it possible to determine the creep stress (Qf), the elastic limit of the soil. Indeed, starting from the final displacement/force curve, the force F _f corresponding to the point of inflection translates said creep stress Qf.

The creep stress is expressed:

with F _f the force corresponding to the point of inflection and S the surface of the plane section of the measuring tip 11.

From the creep stress Qf, one could estimate, for safety, an allowable stress value Qa at ½ Qf, which would place the allowable stress Qa in the middle part of the elastic range E. Recall that the allowable stress Qa is used to the dimensioning of the foundations of the structure.

According to another approach, depending on the allowable settlement of the proposed structure, the allowable stress Qa could be assigned a value, always lower than the creep stress Qf, located in the first elastic range, and corresponding on the curve, at a displacement equal to the known allowable settlement. Remember that traditionally, the allowable stress Qa is evaluated on the basis of a fraction of the breaking stress Qc: Qa = Qc/10; the coefficient 10 being established semi-empirically and in an extremely secure manner. The present method gives a value of the admissible stress Qa based on the management of the admissible settlement vis-à-vis the structure, which makes it much more relevant.

The penetrometer 100 according to the invention therefore allows an in situ measurement to directly reconstitute the action/reaction relationship of a foundation/soil assembly by giving access to the effective behavior of the soil under load: bearing capacity, settlements, creep over time.

Note that the penetrometer 100 according to the second mode of implementation (FIG. 2) will preferably be used for these elasto-plastic measurements, the rigid and short fluidic communication between the second chamber 64 and the first chamber 61 being more suitable for an application effective force/displacement sequences to the piston 62 (in connection with the rod 1) by the adjusting screw 69.

Of course, the invention is not limited to the embodiments described and variant embodiments may be added thereto without departing from the scope of the invention as defined by the claims.

Claims (15)

Penetrometer (100) comprising:
- At least one central rod (1) terminated at a first end by a measuring tip (11);
- At least one hollow tube (2) surrounding the central rod (1), the latter being able to slide inside the hollow tube (2);
- A measuring cell (3) in contact with the hollow tube (2), intended to transmit a force applied by support means (4), so as to cause a depression in static mode in the ground of the hollow tube ( 2) and the central rod (1);
The penetrometer (100) being characterized in that it comprises a compressible system (6) providing an elastic connection between the measuring cell (3) and a second end (12) of the central rod (1), said compressible system ( 6) comprising:
- A first oil chamber (61) formed in the measuring cell (3), and a first piston (62) integral with a second end (12) of the central rod (1) and able to slide in the first chamber (61);
- At least one column (63) offset with respect to the axis of the central rod (1), which comprises a second oil chamber (64) in fluid communication with the first chamber (61), a second piston (65) slidable in the second chamber (64) and a calibrated compressible device (66) in contact with the second piston (65);
- A pressure sensor (67) connected to the second chamber and/or a displacement sensor (68) capable of measuring the length of the calibrated compressible device (66). Penetrometer according to the preceding claim, in which the fluidic communication between the first chamber (61) and the second chamber (64) is made by means of a flexible pipe (614a). Penetrometer according to Claim 1, in which the fluid communication between the first chamber (61) and the second chamber (64) is made by means of a rigid conduit (614b). Penetrometer according to the preceding claim, in which:
- the measuring cell (3) comprises an internal part (3a), an external part (3b) and a sealed annular space (3d) between the internal part (3a) and the external part (3b),
- the first chamber (61) is formed in the internal part (3a) and is able to communicate with the annular space (3d) via a first orifice,
- the rigid conduit (614b) is fixed to the external part (3b) which comprises a second orifice to communicate with the annular space (3d). Penetrometer according to one of the preceding claims, in which the offset column (63) comprises an adjustment screw (69) configured to vary the volume of the second chamber (64) by moving in the offset column (63). Penetrometer according to the preceding claim, in which the adjustment screw (69) is actuated by a motor (70) controlled and controlled by an electronic controller. Penetrometer according to one of the preceding claims, comprising additional driving means (8) able to apply to the hollow tube (2), a vibration at a determined frequency. Penetrometer according to the preceding claim, in which the additional driving means (8) are composed of at least one hydraulic, pneumatic or electric vibro-driving device. Penetrometer according to one of the preceding claims, in which the compressible device (66) is formed by a calibrated spring or by a plurality of Belleville washers. Penetrometer according to one of the preceding claims, comprising the support means (4) capable of applying a force to the measuring cell (3), in order to effect a driving into the ground in static mode of the hollow tube (2) and of the central rod (1). Penetrometer according to one of the preceding claims, comprising a frame (5) provided with at least one mechanical connection element intended to be connected to a reaction mass (200). Penetrometer according to the preceding claim, in which the travel of the measuring tip (11) relative to one end of the hollow tube (2) is defined by the variation in the volume of the second chamber (64) and adapted to the reaction mass ( 200). Use of the penetrometer (100) according to one of the preceding claims, for measuring the penetration resistance of a soil. Use of the penetrometer (100) according to one of Claims 1 to 12, for evaluating the liquefiable nature of a soil. Use of the penetrometer (100) according to one of Claims 1 to 12, for measuring the elastoplastic properties of a soil.